Inflatable corona ring and cable termination method employing same

ABSTRACT

To terminate high voltage cables for testing, an infaltable tubuar annulus such as the inner tube of a tire, but with a corona resistant outer surface, is applied as a corona ring at the line where the shielding is cut from the cable insulation.

United States Patent [191 Lonow [451 Aug. 6, 1974 INFLATABLE CORONA RING AND CABLE TERMINATION METHOD EMPLOYING SAME [75] Inventor: Martin S. Lonow, Spring Valley,

[73] Assignee: The Anaconda Company, New

York, NY.

22 Filed: Sept. 28, 1973 21 Appl. No.: 401,784

[52] US. Cl 174/73 R, 29/592, 174/127, 174/144, 277/34.3, 324/54 [51] Int. Cl... H02g 15/02, GOlr 31/20, HOlt 19/02 [58] Field of Search 174/10, 73 R, 73 SC, 127, 174/140 R, 140 CR, 144; 9/310 F, 345;

128/129, 325, 326, 327, DIG. 20, DIG. 25

[56] References Cited UNITED STATES PATENTS 2,031,870 2/1936 Vertuno 128/327 2,050,888 8/1936 Kirch 174/10 UX 2,230,138 l/l94l Ewart 2,789,154 4/1957 Peterson 174/73 SC 3,038,732 6/1962 Scott et a1 277/34.3 3,190,679 6/1965 Lester 285/97 X 3,455,571 7/1969 Dallera et a1 9/310 F X 3,538,917 11/1970 Selker 128/326 3,730,186 5/1973 Edmunds, Jr. et a1 128/325 FOREIGN PATENTS OR APPLlCATlONS 1,547,264 10/1968 France 174/10 876,507 9/1961 Great Britain 174/73 R 397,017 2/1966 Switzerland 277/34.3

OTHER PUBLICATIONS Draffan, G. L., How to Tame 750-KV with Second-- Hand Tire Tubes, Electrical World, Vol. 155, No. 14, Apr. 3, 1961, page 64.

Primary Examiner-Laramie E. Askin Attorney, Agent, or FirmVictor F. Volk [5 7] ABSTRACT To terminate high voltage cables for testing, an infaltable tubuar annulus such as the inner tube of a tire, but with a corona resistant outer surface, is applied as a corona ring at the line where the shielding is cut from the cable insulation.

5 Claims, 4 Drawing Figures INFLATABLE CORONA RING AND CABLE TERMINATION METHOD EMPLOYING SAME BACKGROUND OF THE INVENTION High voltage cables conventionally comprise an elongated metal conductor, a wall of insulation such as may be built up of a large plurality of layers of oil impregnated paper, surrounding the conductor, and a layer of shielding which, in the case of the paper tape cables, will comprise one or more servings of metal, metallized paper or metallized plastic tape. These may overlie a layer of semiconducting bedding, such as may be formed by servings of carbon-black filled paper. During a test of the cable, the shielding layers will be grounded while the conductor will be energized at high potentials. The conducting and semiconducting shielding is cut off a sufficient distance from the end of the cable and its exposed conductor that the surface resistance will be sufficient to prevent an arc-over from the conductor to the shielding. However, at any practical length of cut-back for the shielding, corona would form at the sharp cut edge of the shielding tapes, except for the practice of building up a stress cone or corona ring at this edge. Conventionally, this ring has been built up by hand wrapping semiconducting tapes, applying a semiconducting coating to a build up of paper tapes, or fitting a tube over the end to be tested, sealing the tube to the cable by means of a wrap, and pouring hot oil into the open end of the tube. An appreciable propor tion of the cost of testing has resided in the hand labor for building taped corona rings and for removing them when the test was completed.

SUMMARY I have invented an improved test method and corona ring which can be applied to cables of different sizes and reused repeatedly with the expenditure of only a small fraction of the time formerly required. My corona ring for terminating a shield of high voltage cable comprises an inflatable rubber-like tubular annulus having an inner diameter sufficient to fit easily over the shield when the tubular annulus is deflated and means limiting the outer diameter when the tubular annulus is inflated, so that the tubular annulus will tighten down over the cable. The tubular annulus has a corona resistant surface which may advantageously be achieved by having the tubular annulus comprised of a corona resistant rubber, by wrappings of corona resistant rubber tape, or by an adherent corona resistant coating. Means for limiting the expansion of the outer diameter of my tubular annulus upon inflation may advantageously comprise a nonstretchable outer circular section of the wall of the tubular annulus, a circumferential tape bonded over the outer surface of the tubular annulus or a substantially circular rigid rim into which the tubular annulus is confined.

My method of terminating a high voltage cable comprising a conductor, a wall of insulation surrounding the conductor and a layer of electrical shielding covering the wall comprises steps of removing the shielding from an end length of the cable to expose a dielectric surface of the insulation having high electrical surface resistance, extending from an end of the cable to a termination of the shielding, fitting an inflated tubular annulus having a corona resistant surface over the cable end, and inflating the tubular annulus at the termina- BRIEF DESCRIPTION OF THE DRAWING FIG. 1 shows a pictorial view of my corona ring being used in the method of my invention.

FIG. 2 shows a pictorial view of one embodiment of my invention.

FIG. 3 shows a section of another embodiment of my invention.

FIG. 4 shows a partial section of another embodiment of my invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Referring to FIG. 1, a high voltage cable 11 having a conductor 12, a thick wall of insulation 13, and a layer of shielding 14 is to be tested by the application of an appropriately high potential to the conductor 12. The shielding layer 14, including any semiconducting component, has been stripped back to the cut termination 16 to leave a long length of the surface of the insulation 13 exposed. In the illustrated example, the insulation 13 is comprised of saturated paper tapes but my invention will also have application to synthetic extruded walls of insulation in which case the surface of the exposed length will be cleaned and possibly abraded to provide a surface of high resistivity free from contamination. An inflatable tubular annulus 17 in its inflated condition is fitted over the cable 11 at the section 16 in electrical contact with the shielding 14. The tubular annulus 17 has a valve 18 and is confined within a rigid rim 19 which may be metal or plastic. The tubular annulus 17 is made of a corona resistant rubber, such, for example as neoprene, butyl, chlorosulfonated polyethylene, ethylene propylene terpolymer (EPDM), and ethylene propylene copolymer (EPM), but alternative materials for the tubular annulus are illustrated in FIGS. 2 and 3. The inside diameter 21 (FIG. 2) of the tubular annulus 17 is large enough in its deflated condition to fit easily over the cable 11 and normally, with the tubular annulus uniformly constructed of a stretchable material, such as rubber, this inside diameter would increase upon inflation along with an increase in the outer diameter. Limiting the expansion of the outer diameter by means of the rim 19, forces the inside wall of the tubular annulus inwardly to tighten against the cable and make a firm circular contact, free from air gaps, with the cable at the termination 16 of the shielding 14. Instead of the rim 19, which is a separate member, I have bonded a relatively nonstretchable tape 22 around the outer surface of a tubular annulus 23 in FIG. 3. The tubular annulus 23 is rubber to which has been applied a corona resistant rubber paint layer 24. In practice, I have also found that an inner tube of a small tire such as a snowmobile tire, can be made suitable for use as a corona ring by moving the tire valve 18 from the inner surface to a sidewall and wrapping the inner tube (FIG. 2) with commercially available corona resistant tape 26. This tape may be semiconducting, with a surface resisitivity of about 5,000 ohms per square, and I believed, initially, that such low surface resistivities were necessary to prevent corona discharges during testing. I have found, however, that'the tapes 26 or, in the absence of tapes, such as in the embodiment of FIG. 1, the mate rial of the annulus 17, need not be semiconducting so long as they were highly resistant to corona degradation.

EXAMPLE A standard size 410/350 X 4 inner tube was partially inflated and the outer circumference wrapped with a glass-reinforced, nonstretchable neoprene filled tape to restrain outward expansion. The inner tube, including the glass reinforced tape, was then helically wound with a vulcanizable neoprene tape, having a volume resisitivity of about 300 X ohms per cm. A fabric overwrap was applied to maintain pressure and the tube was vulcanized at 300 F for 1 hour. After cooling, the fabric overwrap was removed. A high voltge cable was tested with the deflated tube placed over the shielding termination and then inflated to a pressure fit. Corona discharge at the termination was found to be greatly reduced. This reduction of corona discharge was also accomplished by first inflating the tube so that its inner diameter was slightly smaller than the outer cable diameter and then force fitting it over the cable.

In FIG. 4, l have illustrated the sectional structure of a tubular annulus 27, having a relatively nonstretchable section 28 built integrally into the wall of the outer diameter, such as by the incorporation of textile fibers molded in a corona resistant rubber wall. By means of the rim 19, tape 22 or section 28, a tubular annulus can be made to expand inwardly upon inflation and this has the advantage that a single size of tubular annulus can be snugly fitted into firm contact with cables of different diameters. By selecting a tubular annulus constructed of highly resilient material and having a deflated inner diameter slightly less than that of the cable, the tubular annulus can be stretched to fit it over the cable and then inflated without loss of contact of its inner surface with the surface of the cable. To avoid disarrangement of the cable tapes upon fitting a tubular annulus over the cable end or possible scraping off of low resistivity material onto the cable surface, I prefer, however, to use a tubular annulus that will fit over the cable loosely in its deflated condition.

The foregoing description has been exemplary rather than definitive of my invention for which I desire an award of letters Patent as defined in the appended claims.

1 claim:

1. A corona ring for terminating a shield of a high voltage cable comprising:

A. an inflatable rubber-like tubular annulus,

va. said tubular annulus having an inner diameter sufficient for fitting over said shield, said tubular annulus being deflated,

b. said tubular annulus having an outer surface highly resistant to degradation by corona,

c. substantially said entire surface comprising a wrapping of corona resistant tape, and

B. means limiting the outer diameter of said tubular annulus whereby said tubular annulus will tighten over said cable upon inflation.

2. A corona ring for terminating a shield of a high voltage cable comprising:

A. an inflatable rubber-like tubular annulus,

a. said tubular annulus having an inner diameter sufficient for fitting over said shield, said tubular annulus being deflated,

b. said tubular annulus having an outer surface highly resistant to degradation by corona,

c. substantially said entire surface comprising an adherent corona resistant coating, and

B. means limiting the outer diameter of said tubular annulus whereby said tubular annulus will tighten over said cable upon inflation.

3. The method of terminating a high voltage cable comprising a conductor, a wall of insulation surrounding said conductor, and a layer of electrical shielding covering said wall, comprising the steps of:

A. removing said shielding from an end length of said cable, thereby exposing a dielectric surface of said insulation having high electrical surface resistance extending from an end of said cable to a termination of said shielding,

B. fitting an inflatable tubular annulus having a corona resistant surface over said end, positioning said annulus at a location where, when inflated, it will contact said shielding, and

C. inflating said tubular annulus at said termination thereby bringing said surface in contact with said shielding at said termination.

4. The method of claim 3 comprising the step of fitting a rigid rim over said tubular annulus during the inflation thereof.

5. The method of terminating a high voltage cable comprising a conductor, a wall of insulation surrounding said conductor, and a layer of electrical shielding covering said wall, comprising the steps of:

A. removing said shielding from an end length of said cable, thereby exposing a dielectric surface of said insulation having high electrical surface resistance, extending from an end of said cable to a termination of said shielding,

B. inflating a resilient tubular annulus, having a corona resistant surface and means limiting the outer diameter of said annulus, to an inner diameter slightly less than the diameter over said shielding, and

C. passing said annulus over said cable by outwardly compressing said inner diameter to a section of said cable in contact with said shielding at said termination. I 

1. A corona ring for terminating a shield of a high voltage cable comprising: A. an inflatable rubber-like tubular annulus, a. said tubular annulus having an inner diameter sufficient for fitting over said shield, said tubular annulus being deflated, b. said tubular annulus having an outer surface highly resistant to degradation by corona, c. substantially said entire surface comprising a wrapping of corona resistant tape, and B. means limiting the outer diameter of said tubular annulus whereby said tubular annulus will tighten over said cable upon inflation.
 2. A corona ring for terminating a shield of a high voltage cable comprising: A. an inflatable rubber-like tubular annulus, a. said tubular annulus having an inner diameter sufficient for fitting over said shield, said tubular annulus being deflated, b. said tubular annulus having an outer surface highly resistant to degradation by corona, c. substantially said entire surface comprising an adherent corona resistant coating, and B. means limiting the outer diameter of said tubular annulus whereby said tubular annulus will tighten over said cable upon inflation.
 3. The method of terminating a high voltage cable comprising a conductor, a wall of insulation surrounding said conductor, and a layer of electrical shielding covering said wall, comprising the steps of: A. removing said shielding from an end length of said cable, thereby exposing a dielectric surface of said insulation having high electrical surface resistance extending from an end of said cable to a termination of said shielding, B. fitting an inflatable tubular annulus having a corona resistant surface over said end, positioning said annulus at a location where, when inflated, it will contact said shielding, and C. inflating said tubular annulus at said termination thereby bringing said surface in contact with said shielding at said termination.
 4. The method of claim 3 comprising the step of fitting a rigid rim over said tubular annulus during the inflation thereof.
 5. The method of terminating a high voltage cable comprising a conductor, a wall of insulation surrounding said conductor, and a layer of electrical shielding covering said wall, comprising the steps of: A. removing said shielding from an end length of said cable, thereby exposing a dielectric surface of said insulation having high electrical surface resistance, extending from an end of said cable to a termination of said shielding, B. inflating a resilient tubular annulus, having a corona resistant surface and means limiting the outer diameter of said annulus, to an inner diameter slightly less than the diameter over said shielding, and C. passing said annulus over said cable by outwardly compressing said inner diameter to a section of said cable in contact with said shielding at said termination. 